Nearly 1 in 10 San Francisco Muni riders boards a bus or streetcar without paying, costing the city's transit agency an estimated $19 million a year in lost revenue.

The system averages almost 700,000 boardings a day.

The findings, outlined in a study to be presented to the Municipal Transportation Agency governing board Tuesday, echoes what many Muni regulars long have griped about: Fare cheating is commonplace.

Previous studies also reflected that fare evasion is enshrined in Muni culture.

"As essential as Muni is to San Franciscans, there is a widespread perception that many of its customers do not pay their fare to ride the system," the new study found. "This perception has a negative impact on (Muni), reducing public confidence in the system and making it more difficult to increase public funding and implement new initiatives for service improvements."

Muni, faced with a $129 million deficit heading into the new fiscal year that began July 1, raised fares and is preparing to cut service in early December. The regular fare now is $2.

Fare box revenue is expected to generate approximately $170 million in the current fiscal year, accounting for about 20 percent of Muni's operating budget.

Fare evasion on Muni occurs most frequently in the afternoon and at night, the study found. Among the lines where the problem is most prevalent are the 9-San Bruno, 14-Mission, 38-Geary and 47-Van Ness, but few are immune.

It occurs on crowded buses and near-empty ones. In many cases, riders board illegally through the back door; in others they ask the drivers for a "courtesy ride" or refuse to pay.

"I see it all the time, and it makes me mad," said Kathy Hayward, 34, a graphic designer who pays $55 a month for a Fast Pass. "Muni should do a better job collecting what it's owed."

Many drivers reported that they do not feel comfortable forcing people to pay fares "for fear of being assaulted," the study found.

About half the people riding for free did not possess a valid "proof of payment," such as pass or transfer, or were presumed not to have one because they left the scene when they spotted a fare inspector, the study found.

About a quarter of the fare evaders had an expired, altered or illegally obtained transfer, pass or receipt; approximately

7 percent were improperly using a senior or youth pass; and the remainder had invalid proof of payment for a variety of other reasons.

Failure to pay for a ride carries a $75 fine.

The study follows a critical audit by the Board of Supervisors' budget analyst in the spring that found flaws with Muni's proof-of-payment program, citing inefficiencies and questioning its effectiveness.

Since then, Muni began redeploying more of its fare inspectors from the Metro trains to the more problematic buses and altering their work schedules, said Muni spokesman Judson True. The Police Department also has agreed to put more officers on Muni as part of their routine patrols.

In collecting data for the study, surveyors rode more than 1,100 buses and streetcars between late April and late July at different times of the day and every day of the week. They did not track fare collection on cable cars.

City transportation chief Nathaniel Ford, who called the study "groundbreaking," pledged action. "Fare evasion is simply not fair to our paying customers," he said, "and we are doing everything we can to stop it."